1. Field of the Invention
The present invention relates generally to microwave circuits and to circuits utilizing ferrite junction technology, and more particularly to a method of embedding ferrite devices in alumina substrates.
2. Description of the Prior Art
Microwave integrated circuits have been used to produce oscillators, mixers, modulators, and circulators and other devices constructed by combinations of microstrip lines and ferrite disc elements embedded in a non-conductive substrate. For thin film microcircuits, a precise machining technique is required to reduce high frequency losses. One form of the prior art consists of a ferrite disc embedded in a dielectric substrate. The substrate preferably may be comprised of alumina (aluminum oxide, Cl.sub.2 O.sub.3). Reflection and thermal losses of the connections depend primarily on the dimensional precision. Dimensional imperfections, such as the gap between the ferrite disc and hole in the substrate, the roughness of the sidewall, and the roundness of the disc have been found to cause excessive insertion losses.
One such device is described by Ogawa, et al., A 26-GHz Band Integrated Circuit of a Double-Balanced Mixer and Circulators, Trans IEEE MTT, Vol. MTT-30 , No. 1, January 1982. The fabrication process described therein called for preparing a tapered ferrite disc, metalizing the top and bottom surfaces with a gold layer, and embedding by a force fit in a laser drilled, diamond polished tapered hole in the substrate. One disadvantage of this process was that the substrate and ferrite thickness must be at least 0.012 inches or greater, lowering the realizable frequency of operation. Further, it is difficult to obtain a perfect fit between the tapered ferrite disc and substrate hole, particularly with temperature variations. Any variation in the angle of the taper between the hole and the ferrite disc will be observed as an electrical loss. If the hole is not large enough, the pressure used during the embedding process can crack either the ferrite or the substrate; if the hole is too large, there will be a discontinuity caused by the gap between the substrate and ferrite. Further, the temperature coefficient of expansion (TCE) mismatch between the ferrite and substrate can result in microscopic cracks and decreased electrical performance over temperature variations. Moreover, where the holes are laser drilled after metalization and etching, the vaporized metal may produce conductive tracks through the hole. Thus, an additional step of reaming the holes by conventional means is required to clear electrical short circuits.
The present invention provides a method of bonding the ferrite disc to a dielectric substrate that permits relatively large gaps between the ferrite and sidewalls of the substrate but results in very small electrical losses. A bonding composition which matches the temperature coefficients of expansion between the substrate and the ferrite as well as dielectric constant differences is applied to fill in the gaps. The process allows the ferrite to be metalized after embedding, thereby avoiding the problem of vaporized metal tracks in the substrate.